Radio transmitters are used to generate the modulated signals required for wireless communications using modulation techniques such as QPSK, 8-PSK, 16-QAM, 64-QAM, and OFDM to vary the amplitude, phase, and/or frequency of the transmitter's RF carrier.
The modulated signal represents and conveys the message data consisting of in phase (I) and quadrature (Q) data streams. In practice, these data streams pass through digital filters that shape the resulting pulses and ultimately define the spectrum of the modulated transmit signal. A polar transmitter translates these I and Q data streams to equivalent amplitude (AM) and phase (PM) modulation signals. This allows these signals to be applied at more advantageous points in the transmitter increasing its efficiency.
The PM signal is applied to the RF carrier at a phase-locked loop (PLL). In practice, this is actually accomplished using the equivalent frequency modulation (FM) signal, which is easily found by differentiating the PM signal. Unfortunately, the differentiation process widens the bandwidth of the FM signal and also generates impulses. This is due to the fact that the phase jumps by as much as π whenever the transmit signal passes through or near the origin of the complex plane as shown in FIG. 1. The resulting FM impulses (that occur after differentiating the phase jumps), although infrequent, can be as strong as one-half of the data rate.
The FM signal's impulses and wide bandwidth present daunting challenges to the design of the polar transmitter. Any distortion of the FM signal alters the spectrum of the VCO output, elevates the noise floor around the transmit signal, and rotates the complex signal pattern. Practical circuits invariably reduce the bandwidth of the FM signal and degrade performance. More importantly, the VCO and PLL limit the peak FM deviation and corrupt the transmit output spectrum. It would therefore be advantageous to reduce the peak FM deviation as well as the bandwidth of the FM signal.